As is known, systems for harvesting energy (also known as “energy-harvesting systems” or “energy-scavenging systems”) from intermittent environmental energy sources (i.e., sources that supply energy in an irregular way) have aroused and continue to arouse considerable interest in a wide range of technological fields. Typically, energy-harvesting systems are configured to harvest, store, and transfer energy generated by mechanical sources to a generic load of an electrical type.
Low-frequency vibrations, such as, for example, mechanical vibrations of disturbance in systems with moving parts, can be a valid source of energy. The mechanical energy is converted by one or more appropriate transducers (for example, piezoelectric or electromagnetic devices) into electrical energy, which can be used for supplying an electrical load. In this way, the electrical load does not require batteries or other supply systems that are cumbersome and poorly resistant to mechanical stresses.
FIG. 1 is a schematic illustration, by means of functional blocks, of an energy-harvesting system of a known type.
The energy-harvesting system 1 of FIG. 1 comprises: a transducer 2, for example of an electromagnetic or piezoelectric type, subject during use to environmental mechanical vibrations and configured for converting mechanical energy into electrical energy, typically into AC (alternating current) voltages; a scavenging interface 4, for example comprising a diode-bridge rectifier circuit (also known as Graetz bridge), configured for receiving at input the AC signal generated by the transducer 2 and supplying at output a DC (direct current) signal for charging a capacitor 5 connected to the output of the rectifier circuit 4; and a DC-DC converter 6, connected to the capacitor 5 for receiving at input the electrical energy stored by the capacitor 5 and supplying it to an electrical load 8. The capacitor 5 hence has the function of energy-storage element, energy which is made available, when required, to the electrical load 8 for operation of the latter.
The transducer 2 is, for example, an electrochemical transducer, or an electromechanical transducer, or an electroacoustic transducer, or an electromagnetic transducer, or a photoelectric transducer, or an electrostatic transducer, or a thermoelectrical transducer.
The global efficiency ηTOT of the energy-harvesting system 1 is given by Eq. (1) belowηTOT=ηTRANSD·ηSCAV·ηDCDC  (1)
where: ηTRANSD is the efficiency of the transducer 2, indicating the amount of energy available in the environment that has been effectively converted by the transducer 2 into electrical energy; ηSCAV is the efficiency of the scavenging interface 4, indicating the energy consumed by the scavenging interface 4 and the factor of impedance decoupling between the transducer and the interface; and ηDCDC is the efficiency of the DC-DC converter 6.
As is known, in order to supply to the load the maximum power available, the impedance of the load should be equal to that of the source. The transducer 2 can be represented schematically, in this context, as a voltage generator 3 provided with an internal resistance RS of its own. The maximum power PTRANSDMAX that the transducer 2 can supply at output may be defined as:PTRANSDMAX=VTRANSD_EQ2/4RS if RLOAD=RS  (2)
where: VTRANSD_EQ is the voltage produced by the equivalent voltage generator; and RLOAD is the equivalent electrical resistance at the output of the transducer 2 (or, likewise, seen at input to the scavenging interface 4), which takes into due consideration the equivalent resistance of the scavenging interface 4, of the DC-DC converter 6, and of the load 8.
On account of the impedance decoupling (RLOAD≠RS), the power at input to the scavenging interface 4 is lower than the maximum power available PTRANSDMAX.
The power PSCAV stored by the capacitor 5 is a fraction of the power recovered by the interface, and is given by Eq. (3) belowPSCAV=ηTRANSD·ηSCAV·PTRANSDMAX  (3)
while the power PEL_LOAD supplied at output by the DC-DC converter to the electrical load 8 is given by the following Eq. (4)PEL_LOAD=PDCDC·ηDCDC  (4)
where PDCDC is the power received at input by the DC-DC converter 8, in this case coinciding with PSCAV.
The main disadvantage of the configuration according to FIG. 1 regards the fact that the maximum voltage supplied at output from the scavenging interface 4 is limited by the input dynamics of the DC-DC converter 8.
The voltage VOUT across the capacitor 5 (supplied at output from the scavenging interface 4 and at input to the DC-DC converter 8) is in fact determined on the basis of the balancing of power according to the following Eq. (5)PSTORE=PSCAV−PDCDC  (5)
where PSTORE is the excess power with respect to what is required by the load, recovered by the harvesting interface 4 and stored in the capacitor 5.
In applications where the transducer 2 converts mechanical energy into electrical energy in a discontinuous way (i.e., the power PTRANSDMAX varies significantly in time) and/or the power PEL_LOAD required by the electrical load 8 varies significantly in time, also the voltage VOUT consequently presents a plot that is variable in time.
This causes, for example, a variation of the efficiency factor ηDCDC which assumes low values at high values of VOUT. The maximum value of VOUT is moreover limited by the range of input voltages allowed by the DC-DC converter.
European Patent Application No. EP 2518878, incorporated herein by reference, describes a DC-DC converter that makes it possible to maintain an efficiency factor ηDCDC high even in a condition of light load and to obtain a high dynamic of input voltages. The DC-DC converter according to the document EP 2518878 is of the SIMO (single-inductor multiple-output) type, and is configured to supply a plurality of electrical loads. However, such a DC-DC converter presents some limitations. For example, the supply of the electrical loads follows a fixed-time multiplexing procedure, which envisages a pre-set sequence of supply of the electrical loads. Moreover, each load is supplied during a respective time slot, the duration of which is defined by a pre-set clock signal independent of the load that is being supplied.